This invention relates to the preparation of capsules, especially capsules intended for use in forming electrophoretic media.
Particle-based electrophoretic displays, in which a plurality of charged particles move through a suspending fluid under the influence of an electric field, have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays.
(The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in published U.S. Patent Application No. 2002/0180687 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.)
Nevertheless, problems with the long-term image quality of electrophoretic displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspending medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828; 6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374; 6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524; 6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997; 6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075; 6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540; 6,721,083; 6,727,881; 6,738,050; 6,750,473; 6,753,999; 6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970; 6,831,769; and 6,839,158; and U.S. Patent Applications Publication Nos. 2002/0060321; 2002/0063661; 2002/0090980; 2002/0113770; 2002/0130832; 2002/0131147; 2002/0171910; 2002/0180687; 2002/0180688; 2003/0011560; 2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908; 2003/0137521; 2003/0151702; 2003/0214695; 2003/0214697; 2003/0222315; 2004/0008398; 2004/0012839; 2004/0014265; 2004/0027327; 2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681; and 2004/0196215; and International Applications Publication Nos. WO 99/67678; WO 00/05704; WO 00/38000; WO 00/38001; WO00/36560; WO 00/67110; WO 00/67327; WO 01/07961; WO 01/08241; WO 03/107,315; WO 2004/023195; WO 2004/049045; WO 2004/059378; WO 2004/088002; WO 2004/088395; WO 2004/090857; and WO 2004/099862.
Known electrophoretic media, both encapsulated and unencapsulated, can be divided into two main types, referred to hereinafter for convenience as “single particle” and “dual particle” respectively. A single particle medium has only a single type of electrophoretic particle suspended in a suspending medium, at least one optical characteristic of which differs from that of the particles. (In referring to a single type of particle, we do not imply that all particles of the type are absolutely identical. For example, provided that all particles of the type possess substantially the same optical characteristic and a charge of the same polarity, considerable variation in parameters such as particle size and electrophoretic mobility can be tolerated without affecting the utility of the medium.) When such a medium is placed between a pair of electrodes, at least one of which is transparent, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of the particles (when the particles are adjacent the electrode closer to the observer, hereinafter called the “front” electrode) or the optical characteristic of the suspending medium (when the particles are adjacent the electrode remote from the observer, hereinafter called the “rear” electrode (so that the particles are hidden by the suspending medium).
A dual particle medium has two different types of particles differing in at least one optical characteristic and a suspending fluid which may be uncolored or colored, but which is typically uncolored. The two types of particles differ in electrophoretic mobility; this difference in mobility may be in polarity (this type may hereinafter be referred to as an “opposite charge dual particle” medium) and/or magnitude. When such a dual particle medium is placed between the aforementioned pair of electrodes, depending upon the relative potentials of the two electrodes, the medium can display the optical characteristic of either set of particles, although the exact manner in which this is achieved differs depending upon whether the difference in mobility is in polarity or only in magnitude. For ease of illustration, consider an electrophoretic medium in which one type of particles is black and the other type white. If the two types of particles differ in polarity (if, for example, the black particles are positively charged and the white particles negatively charged), the particles will be attracted to the two different electrodes, so that if, for example, the front electrode is negative relative to the rear electrode, the black particles will be attracted to the front electrode and the white particles to the rear electrode, so that the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, the white particles will be attracted to the front electrode and the black particles to the rear electrode, so that the medium will appear white to the observer.
If the two types of particles have charges of the same polarity, but differ in electrophoretic mobility (this type of medium may hereinafter to referred to as a “same polarity dual particle” medium), both types of particles will be attracted to the same electrode, but one type will reach the electrode before the other, so that the type facing the observer differs depending upon the electrode to which the particles are attracted. For example suppose the previous illustration is modified so that both the black and white particles are positively charged, but the black particles have the higher electrophoretic mobility. If now the front electrode is negative relative to the rear electrode, both the black and white particles will be attracted to the front electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the front electrode and the medium will appear black to the observer. Conversely, if the front electrode is positive relative to the rear electrode, both the black and white particles will be attracted to the rear electrode, but the black particles, because of their higher mobility will reach it first, so that a layer of black particles will coat the rear electrode, leaving a layer of white particles remote from the rear electrode and facing the observer, so that the medium will appear white to the observer: note that this type of dual particle medium requires that the suspending fluid be sufficiently transparent to allow the layer of white particles remote from the rear electrode to be readily visible to the observer. Typically, the suspending fluid in such a display is not colored at all, but some color may be incorporated for the purpose of correcting any undesirable tint in the white particles seen therethrough.
Both single and dual particle electrophoretic displays may be capable of intermediate gray states having optical characteristics intermediate the two extreme optical states already described.
Some of the aforementioned patents and published applications disclose encapsulated electrophoretic media having three or more different types of particles within each capsule. For purposes of the present application, such multi-particle media are regarded as sub-species of dual particle media.
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, the aforementioned 2002/0131147. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, the aforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat. Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Other types of electro-optic displays may also be capable of operating in shutter mode.
An encapsulated or microcell electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
The preferred process for preparing electrophoretic capsules described in the aforementioned E Ink and MIT patents and applications uses a gelatin/acacia coacervate as the encapsulation material, and the process for forming such gelatin/acacia capsules may be summarized as follows; see, for example, the aforementioned 2002/0180687, Paragraphs [0069] to [0074]. An internal phase is prepared containing one or more types of electrophoretic particles in a suspending fluid; typically, the internal phase comprises titania and carbon black particles in an uncolored hydrocarbon suspending fluid. The internal phase is thoroughly stirred to ensure that it is homogeneous. Gelatin is dissolved in deionized water at a temperature of 40° C., and vigorously stirred. The internal phase, heated to the same temperature, is added dropwise to the stirred gelatin solution through a tube the outlet of which is below the surface of the stirred solution. The resultant mixture is held at 40° C. with continued vigorous stirring to produce a dispersion of droplets of the internal phase in a continuous gelatin-containing aqueous phase.
A solution of acacia in water at 40° C. is then added to the mixture, and the pH of the mixture is lowered to approximately 4.9 to cause formation of the gelatin/acacia coacervate, thereby forming capsules. The temperature of the resultant mixture is then lowered to 10° C. and an aqueous solution of glutaraldehyde (an agent for cross-linking the capsule walls) is added. The resultant mixture is then warmed to 25° C. and stirred vigorously for a further 12 hours.
The capsules produced are separated from the liquid and washed by redispersion in water. The capsules are then separated by size by sieving or otherwise. For reasons explained in several of the aforementioned E Ink and MIT patents, it is desirable that an encapsulated electrophoretic medium comprises a single, substantially close-packed layer of capsules. Also, when such an electrophoretic medium is produced by coating capsules on to a substrate, it is desirable that the exposed surface of the capsule layer be reasonably flat, since otherwise difficulties may be encountered in laminating the capsule layer to other layers in the final display. Production of such a substantially close-packed layer with a reasonably flat exposed surface is best achieved by coating capsules which are of substantially the same size. Typically, the desired range of capsule diameters will be 30-50 μm, with an average of 40 μm.
Unfortunately, it has been found that the prior art encapsulation process described above does not produce a high yield of capsules with the desired range of diameters. Typically, the yield of the desired capsules (measured as volume percent of the total capsules produced) ranges from 12 to 34 percent, with an average of about 27 percent. Although the average capsule diameter can be varied over a considerable range by controlling parameters such as gelatin concentration, stirring rate, etc., it has not proved possible to increase yields above this range, the major problem being the high proportion of “fines” or capsules having diameters below the acceptable range; for example, as shown in the Example below, in a typical optimized prior art process designed to produce 30-50 μm capsules, the volume percent capsules actually peaks at about 10-12 μm. Obviously such low yields of useful capsules are a major problem, since they greatly increase the cost of the electrophoretic medium and generate substantial costs for disposing of the unacceptable capsules. The prior art process is also not easily scalable, requiring re-optimization each time the size and/or shape of the reactor is changed.
The present invention provides two different approaches to overcoming the aforementioned disadvantages of the prior art encapsulation process, and thus substantially increasing the yield of useful capsules. The first approach involves a relatively straightforward modification of the prior art process in which the coacervate is first formed and thereafter the material to be encapsulated is emulsified in this coacervate phase. The second approach represents a more fundamental change in the process using a limited coalescence (“LC”) process.
Techniques for capsule formation other than stirring of one phase into another are known. One group of such techniques, which have been developed primarily for the preparation of droplets of monomer for use in suspension polymerization processes, are limited coalescence processes; in such processes, the increased particle size uniformity and suspension volume fraction provided by limited coalescence processes allow more efficient use of equipment. There is an extensive patent literature regarding such processes; see, for example, U.S. Pat. Nos. 4,965,131 and 2,932,629.
Limited coalescence processes involves the use of so-called Pickering emulsions for the stabilization of droplets at the critical stage of the process. Pickering emulsions are emulsions (either oil-in-water or water-in-oil) stabilized against coalescence by surface-active particulate materials (also referred to as “particulate colloids” or “PC's”), rather than, as in the case of most conventional emulsions, small surfactant molecules or surface-active soluble polymers. Typical surface-active particulate materials include clay particles, colloidal silica dispersions, and lightly cross-linked latex particles, though many other materials have also been used. Since most of these materials are not intrinsically surface active (that is, they will not partition specifically to an oil-water interface), a second component, called a promoter, is required to achieve stable Pickering emulsions. Promoters are typically themselves somewhat surface active; examples include amphiphilic species of many types, including typical surfactants (soaps, alkyl sulfonates, alkyl ammonium salts, and the like), as well as soluble polymers (gelatin, poly(vinylpyrrolidone), poly(ethylene oxide), poly(vinyl alcohol), etc.) and oligomers (e.g., the condensation product of alkylaminoethanol and adipic acid and similar species as described in the aforementioned patents). The promoter is believed to function by adsorbing on the surface-active particulate material, thus rendering it surface active at the oil-water interface. The amount of promoter is therefore very important, since good promoters are generally materials that will destabilize the PC. If too little promoter is present, the surface-active particulate material will not adsorb at the oil-water interface, and the oil droplets will not acquire stability; however, if too much promoter is present, the surface-active particulate material will either coagulate and separate from the suspension, or partition into the oil phase. A large excess of promoter is, in many cases, capable of stabilizing a conventional emulsion at the expense of a Pickering emulsion. In each of these cases, the result is failure of the limited coalescence process, which requires the formation of a stable Pickering emulsion.
In a typical limited coalescence process, an oil phase is suspended in an aqueous suspension comprising water, surface-active particulate material, and promoter. The resulting crude emulsion is then homogenized under conditions expected to yield a very small particle-sized oil-in-water emulsion. This emulsion is unstable, because the small size of the particles makes the oil-water interfacial area very much larger than that which can be covered by the limited amount of surface-active particulate material present. Coalescence therefore occurs, with a concomitant reduction in interfacial area. As this area decreases, the fraction of the interfacial area covered by adsorbed particulate material increases, and as this fraction approaches 1 (complete coverage) coalescence stops (hence the term “limited coalescence” for the process as a whole). The process has been studied theoretically and mechanistically (see, for example, Whitesides and Ross, J. Interface Colloid Sci. 196, 48-59 (1995)). Monte Carlo simulations have shown that, with reasonable assumptions concerning the probability that collision between droplets will result in coalescence (in particular that the probability of coalescence is a decreasing function of the fractional coverage of the interface by the particulate material), the final droplet particle size distribution will be quite narrow. It is found that the volume of the largest droplets will be approximately twice the volume of the smallest, so that the particle diameters vary by only about the cube root of 2 (approximately 1.26). The best limited coalescence processes match this expectation well.
As already mentioned, limited coalescence processes have primarily been developed for the preparation of droplets of monomer for use in suspension polymerization processes, and there are a number of problems with applying such processes in other contexts. Firstly, a particulate colloidal stabilizer must be found that is compatible with the particular system in which it is to be used. Secondly, the oil-in-water emulsion formed during homogenization must be unstable with respect to coalescence, and this requires that any surfactants already present in the system must be rendered ineffective.
It has now been found that these problems can be overcome and limited coalescence processes used successfully for the production of capsules for use in electrophoretic displays with substantially improved yields as compared with the prior art capsule production processes described above.